[textprocessing] Fix: Accept scheduling a task if there are equivalent taskprocessing providers only

[julien-nc]

Runtask and RunOrSchedule are not affected.

Should be backported to stable30.

To reproduce the issue:

curl -H "ocs-apirequest: true" -u admin:admin \
    http://nextcloud.local/ocs/v2.php/textprocessing/schedule \
    -X POST \
    -d '{"input":"bla and bla but bla","type":"OCP\\TextProcessing\\SummaryTaskType","appId":"plopapp"}' \
    -H "Content-Type: application/json"

while having no text processing providers but only task processing core:text2text:summary providers.

[marcelklehr]

lol

[julien-nc]

/backport to stable30